Chore: split monster-sized modules in the codebase

We have a bunch of modules that are so big that they cause code editors to hang. The champions are probably in the compiler project, for example CheckExpressions (>10,000 LOC) or CheckDeclarations (>5,000 LOC).

We should take some monster every sprint and cut it into parts.

Start by inspecting the compiler modules CheckExpressions (>10,000 LOC) and CheckDeclarations (>5,000 LOC), then identify one monster to target in a sprint. Define the parts to extract and the editor-hang improvement to verify; the issue provides no file paths, tests, or concrete completion criteria.

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